How to upgrade from 1.9 to 2.0?

Backup your website.

Upgrade and test it on a staging server (test environment with a copy of your current site for development purpose) if your hosting provider has this feature. You can work on a live site too, without staging, but at least create a backup. You are not going to loose any content or data, but it's always good to have backup.

Fix issues and conflicts step by step on staging, we will help you if any problems.